In many drive systems operatively connecting a prime mover to a load moved by the drive system, it is necessary to provide a no-back device. The primary function of these no-back devices is to prevent an aiding load from over-running the drive, and/or prevent an opposing load from reversing the drive. Some no-backs also function to provide a small amount of braking during operation of the drive to dampen load oscillations.
Drive systems requiring such no-back functions are commonly found in winches, and in actuation systems for movable aircraft flight control surfaces, such as flaps, slats, or horizontal stabilizers. In these systems, it is essential that the driven load remain as positioned by the drive system, despite the forces of gravity, or aerodynamic buffeting which create aiding or opposing loads. For example, a winch must be capable of reliably holding a load in an elevated position, and of lowering the load in a controlled manner in order to be useful, and to prevent the possibility of damage to the load or injury to operating personnel. An aircraft control surface must remain in the position desired by the flight crew, or the flight characteristics of the aircraft will be seriously degraded.
A typical no-back device includes a releasable brake acting on a shaft operably connected to the drive apparatus. If the no-back is required to be bi-directional, a pair of releasable brakes will generally be utilized, with one brake resisting unwanted motion of the shaft, in one direction, and the second brake resisting unwanted shaft motion in the opposite direction.
Although many types of hydraulic, pneumatic, electrical, or mechanical devices have been utilized to activate/deactivate the releasable brakes in no-back devices, simple mechanical devices are often preferred due to their ruggedness, reliability, low cost, and small size and weight.
In one commonly used mechanical activation/deactivation approach, a rachet and pawl mechanism is utilized to actuate a brake acting on a surface of the shaft. The ratchet wheel is mounted to rotate about a common axis with the shaft. The pawl mechanism is generally pivotably or slidably mounted in a housing surrounding the shaft and the rachet wheel. A series of ratchet teeth on the ratchet wheel, and the pawl mechanism are configured such that the ratchet wheel may rotate freely in one direction, with the pawl ratcheting across, but not engaging, the rachet teeth. The ratchet teeth and the pawl are further configured such that the pawl will engage the ratchet teeth and prevent rotation of the ratchet wheel in the opposite direction, however. A friction producing surface or device is provided between the ratchet wheel and the shaft such that when the shaft is rotated in the one direction, the rachet wheel will rotate freely with the shaft. Should the shaft try to rotate in the opposite direction, however, the ratchet and pawl mechanism will lock-up and prevent the ratchet wheel from rotating with the shaft. In order for the shaft to more further, therefore, there would have to be relative motion between the shaft and the ratchet wheel. This motion will be resisted by the friction producing surface or device, however, thereby preventing unwanted motion of the load. Examples of devices utilizing such ratchet and pawl mechanisms are provided in U.S. Pat. Nos.: 4,697,672; 4,834,225; 4,762,205; and 2,653,691.
In some instances it is desired to have a finer degree of control, i.e. less back motion, than can be obtained by a single pawl mechanism. By adding additional pawls, spaced to operate sequentially on different teeth of the ratchet wheel, such finer control can be achieved. U.S. Pat. No. 2,653,691 illustrates such an arrangement. In the '691 patent, a pair of pawls 44 are angularly displaced about a friction ring 42 in such a manner that the pawls 44 will successively engage teeth 43 of the friction ring 42 within an angular movement of the friction ring equal to one-half of the angle between adjacent teeth.
While the no-back devices described above work well in some applications, further improvement is required. Particularly, it is an object of my invention to provide a no-back device which incorporates a secondary, or fail safe, no-back function in addition to the primary no-back function provided by the prior no-back devices described above. This secondary no-back function is essentially dormant as long as the primary no-back elements are functioning properly. Following a failure of the primary no-back function, however, the secondary no-back function of my invention is immediately activated, thereby allowing continued safe operation of the drive apparatus until such time as the primary no-back function can be restored.
By requiring that the secondary no-back features only become active following a failure of the primary no-back features, wear and fatigue of the secondary no-back components is precluded. System reliability is therefore improved in comparison to prior no-back devices which utilized redundant primary no-back features that were fully active during all operating conditions.
Other objects of my invention include: providing annunciation of the failure of the primary no-back function to alert operators and repair personnel; and providing a locking feature, actuated by activation of the secondary no-back function, which will allow the drive mechanism to only drive the load in one direction, i.e. back to a neutral or safe position, following activation of the secondary no-back function.